1. Field of the Invention
The present invention relates to xerographic or electrophotographic machines and processes for producing duplexed copies. More particularly, the present invention relates to electrophotographic printers and copiers which sequentially transfer images to opposite sides of copy sheets. This invention is especially useful in xerographic printers and is described herein within that context. However, the invention is likewise adaptable for use in conjunction with xerographic copiers.
2. Description of the Prior Art
Electrophotographic machines operating in a duplex mode wherein sequential images are transferred to opposite sides of a copy sheet, typically place the first image on the copy sheet and proceed with reversal of the copy sheet before reintroducing it to the image transfer station. An intervening tray or the like provides temporary storage and side reversal of the original copies. Frequently, the duplex tray holds only a single copy sheet but it is possible to accumulate a multiplicity of copy sheets where a multiplicity of copies are made of each image. In addition, it is possible to store multiple copies of sheets having sequential images on one side with feeding of those sheets to the imaging station provided the second side image is presented in proper order. Thus, prior art copiers and printers employ an essentially closed loop return path for the copy sheets intended to receive duplex copying. A logical decision is rendered as the copy sheet leaves the transfer and fusing paths to determine whether that sheet should enter the return loop or exit to a tray, collator, stapler/finisher, or the like. An example of an electrophotographic printer using a closed loop duplex copy sheet path is shown in commonly-assigned U.S. Pat. No. 4,272,181 filed Dec. 29, 1978 by R. C. Treseder.
Automatic duplex copying with use of return loop duplex paths is predominantly useful in electrophotographic machines having relatively high speed and high volume throughput. Accordingly, any delay in completing the copy sequences discounts the throughput speed of the machine. Unfortunately, the prior art electrophotographic machines are not arranged and operated so as to utilize every available transfer cycle associated with a given run. As a result, the throughput of the machine is necessarily reduced. The present invention resolves this problem by effectively arranging and utilizing the electrophotographic machine elements so as to use all available transfer cycles.